Much of the talk about North American collector car auctions this year has been about "slowdown" and "flattening out" as the huge growth we've seen in the previous few years hasn't been sustained in 2015. While this is true, it often obscures the fact that the market is still growing. For example, in terms of combined total sales for the entire year, results are within 1 percent of 2014, and there are three full months and several big auctions left in 2015.

Mecum has four sales left in 2015, while RM Sotheby's has both its Hershey sale next week and a sale in New York City in December. A sizable increase in annual total sales over last year is all but guaranteed, although it likely won't bee as big as the increases seen in 2013 and 2014.

Compared to this time last year, the number of cars offered at auction is up 4.6 percent, while the number of cars sold is down 3.9 percent. The average sale price, though, is up 14.3 percent, so while fewer cars are selling despite more being offered, those that do sell are still achieving higher prices on average than a year ago. Don't confuse deceleration with contraction.